When printed circuits are manufactured by the subtractive or etched process, panels are used which generally comprise an epoxy resin-impregnated woven glass fiber layer commonly known as "prepreg," having bonded on opposite sides thin copper foil. This unitary assembly, frequently called a core, is then coated with a photosensitive resist, suitably exposed and developed, and thereafter etched to produce the desired conductive paths on the surface of the glass fiber layer.
With the advent of film-type photoresists, it has been found necessary to provide a relatively smooth surface on the copper covered cores so that the photoresist film is well secured over the entire surface of the copper and does not bridge depressions under which etching solutions may penetrate. Therefore, during the lamination of the copper foil to the epoxy resin-impregnated glass fiber layer, a smoothly finished steel planishing plate has been used next to the copper foil to achieve the necessary smoothness required of the copper surface.
A press lay-up consists of several loose core assemblies of resin-impregnated glass cloth layers and sheets of metal foil, and planishing plates are inserted between each metal-glass cloth-metal grouping of sheets. The planishing plates occupy a large portion of each press opening and thereby severely limit the capacity of the press to produce printed circuit cores each cycle. After the press openings are filled, the press is actuated to apply pressure to the lay-up and the platens are heated to cure the epoxy resin. The planishing plates heat slowly and absorb large quantities of ineffective energy. This compressing and heating cycle requires approximately a 2-hour period, depending primarily upon the adhesive and temperature used. Thereafter, the press is opened, the now-laminated cores are removed, and the planishing plates are reused with new loose assemblies of resin impregnated glass fiber and foil.
The steel planishing plates have been found to produce dimensional instability of the laminated panels because the steel has a thermal coefficient of expansion different from that of the metal foil or glass fiber and resin layers. Thus, during the curing process, the panel is cured with stresses which are retained and cause instability during subsequent processing steps for the cores or panels. In addition, planishing plates are of significant weight and contribute to operator fatigue during the insertion and removal of the plates when laying up and tearing down press loads. Although the planishing plates are usually made of a durable material such as stainless steel, they eventually became dented or scratched with use and at times have to be resurfaced because these imperfections are transferred to the copper foil.
Accordingly, it is a primary object of this invention to provide a laminating method which obviates the need of planishing plates for surface finishes.
Another important object of this invention is to provide a laminating method which provides improved dimensional accuracy in the laminate produced thereby.
A still further object of this invention is to provide a laminating method which results in greater production of laminated panels with fewer defects thus effecting significant cost savings.
Yet another object of this invention is to provide a multi-step laminating method in which partially cured panels are substituted for planishing plates with loosely laid up sheets that make up a core assembly.